FIG. 1 shows the structure of a digital communication system. As shown in FIG. 1, the digital communication system consists of a transmitter, a channel and a receiver, wherein the transmitter typically includes a source, a source encoder, a channel encoder, a modulator etc., the receiver usually includes a demodulator, a channel decoder, a source decoder, a destination etc., and during the communication process between the transmitter and the receiver, the transmitter transmits data to the receiver through the channel in which a noise source usually exists.
In the digital communication system, a channel coding link (including channel encoding and decoding, modulation and demodulation, etc.) is a key part of the whole digital communication physical layer, and the processing of the channel coding link determines the effectiveness and reliability of a bottom layer transmission in the digital communication system.
FIG. 2 shows a flowchart of information block bit data passing through the channel coding link to output modulation symbols. As shown in FIG. 2, the channel coding link mainly includes several parts of processing as follows.
Channel Coding (CC for Short)
The channel coding enables the system to have the ability of automatically correcting errors by artificially adding redundant information, to combat various kinds of noises and interferences in the transmission process, so as to ensure the reliability of the digital transmission. For example, a Convolutional Turbo code is acknowledged as one of excellent forward error correction (forward-acting error correction) codes, and is widely used in many standard protocols as a channel coding solution of data service transmission.
Rate Matching (RM for Short)
The rate matching processing is a key technique for the follow-up operations of the channel coding, and aims to repeat or puncture the code word bits after the channel coding, wherein the repeating or puncturing operation may be controlled by an algorithm, to ensure that the data bit length after the rate matching matches the allocated physical channel resources.
At present, there are two kinds of rate matching algorithms: 3GPP R6 rate matching algorithm and Circular Buffer Rate Matching (CBRM for short) algorithm, wherein as the Circular Buffer Rate Matching algorithm is capable of generating a simple algorithm with excellent performance of punctured patterns, the Circular Buffer Rate Matching algorithm is used in the 3GPP2 standard series, the IEEE802.16 standard and the 3GPP LTE standard for rate matching.
Hybrid Automatic Repeat Request (HARQ for Short)
The HARQ is an extremely important link adaptive technology in the digital communication system. The implementation process of the technology is: the receiver decoding the received HARQ data packet, if the decoding is correct, returning an ACKnowledge character (ACK for short) to the transmitter to inform the transmitter of transmitting a new HARQ data packet; if the decoding fails, returning a Negative ACKnowledge character (NACK for short) signal to the transmitter to request the transmitter to retransmit the HARQ data packet again. The receiver can improve success probability of decoding and meet the requirement of high reliability in the link transmission, by performing IR or Chase combination decoding on the data packets retransmitted for multiple times.
HARQ Subpacket Identifier (SPID for Short)
The SPID is currently used in the IEEE802.16 standard for determining the specific location of the subpacket data in a circular buffer region.
In an IEEE802.16 system, the HARQ subpacket identifier and an HARQ data packet length collectively define a starting position and length of the HARQ subpacket data in the circular buffer region, so that a section of code words may be selected from the circular buffer region to generate the current HARQ subpacket, wherein the value range of the SPID is {00, 01, 10, 11}, which occupies 2 bits in a control signaling. The SPID value transmitted for the first time must be 00, and the SPID value in a retransmission can be arbitrarily selected or selected in a certain order in the range. That is, in multiple transmissions, a certain SPID value may be repeatedly used, or a certain SPID value may not be used. In particular, when the code rates for the multiple transmissions are the same, and the SPID values are in sequence of 00, 01, 10, 11, the locations of the transmitted subpackets in a mother code are consecutive in turn.
HARQ Subpacket Generation Process
In the IEEE802.16 standard, the HARQ subpacket data are generated by using a method of Circular Buffer Rate Matching (CBRM for short) processing flow, and the method specifically includes the processes as follows.
Assume that there is an information block bit data sequence I (i0, i1, L, iK−1) wherein K is the length of the information block bit data, ik (0≦k≦K−1) is binary bit data, the information block bit data I is performed with a CTC encoding, and the output CTC encoded code word bit stream sequence is C (c0, c1, L c3×K−1) , assuming that the code rate of the mother code of the CTC encoding is 1/3 (here the code rate of the mother code of the CTC encoding of 1/3 is just an example, other code rates may also be used).
Then, a bit separation operation is performed on the output CTC encoded code word bit stream sequence C to obtain a systematic bit stream (or system bit stream, also referred to as systematic bits) sequence S (s0, s1, L sK−1), a first parity bit stream (or first parity bits) sequence P1 (p01, p11, L pK−11) and a second parity bit stream (or second parity bits) sequence P2 (p02, p12, L pK−12).
Then, a sub-block interleaving is performed on the separated systematic bit stream sequence S, the first parity bit stream sequence P1 and the second parity bit stream sequence P2 respectively, to obtain a sub-block interleaved systematic bit stream sequence SI (s0I, s1I, L, sK−1I), a sub-block interleaved first parity bit stream sequence P1I (p10I, p11I, L, p1K−1I), and a sub-block interleaved second parity bit stream sequence P2I (p20I, p21I, L, p2K−1I).
Bit interleaving is performed on the first parity bit stream sequence P1I after sub-block interleaving processing and the second parity bit stream sequence P2I after sub-block interleaving processing, to form a check bit sequence (parity bit sequence) P(p0I, p1I, L, p2K−1I) , wherein the check bit sequence P and the first parity bit stream sequence P1I after sub-block interleaving processing, the second parity bit stream sequence P2I after sub-block interleaving processing respectively satisfy the following relation:p2kI=p1kI(0≦k≦K−1), andp2k+1Ip2kI(0≦k≦K−1).
In the above, a virtual circular buffer CB (cb0, cb1, L, cb3×K−1) is formed according to the sequence that the sub-block interleaved systematic bit stream SI precedes the check bit sequence P, and the virtual circular buffer CB and the systematic bit stream SI after sub-block interleaving processing, the check bit sequence P respectively satisfy the following relation:cbk=skIk=0,1,L K−1, andcbK−k=pkIk=0,1,L 2K−1.
In the IEEE802.16 protocol, a starting position from which the HARQ data is read in the virtual circular buffer is determined according to the subpacket identifier (SPID for short), and the specific formula is pos(SPID)=(L*SPID) mod(3*K), wherein, L is the length of the transmitted HARQ data packet.
Bit data D=(d0, d1L , dL−1) of the transmitted HARQ packet with the size of L is circularly read from the starting position pos(SPID) in the virtual circular buffer.
High Order Modulation
In order to achieve higher frequency spectrum utilization, in a large number of communication standard protocols, it is more and more inclined to use a high order modulation method to improve the frequency spectrum utilization and peak value transmission rate performance of the system, wherein the most commonly used high order modulation methods include 16 QAM, 64 QAM and so on. In these high order modulation methods, constellation point mapping bits often have different levels of reliability, that is, in a single modulation symbol, two bits therein have higher bit error probability than another two bits in the modulation symbol. Therefore, how to use the different reliabilities of the constellation point mapping bits to improve decoding and transmission performance is a problem to be solved currently.
FIG. 3 shows a constellation with a modulation method of 16 QAM in the IEEE802.16 system. As shown in FIG. 3, the reliability of bits b1, b3 is higher than that of bits b0, b2 , so b1, b3 are referred to as high priority bits, and b0, b2 are referred to as low priority bits. The constellation arrangement of 64 QAM in the IEEE802.16 is as shown in FIG. 4, wherein bits b2, b5 have the highest reliability, followed by bits b1, b4, and bits b0, b3 have the poorest reliability, so bits b2, b5 are referred to as high priority bits, bits b1, b4 are referred to as medium priority bits, and bits b0, b3 are referred to as low priority bits.
Constellation Re-Arrangement (CoRe for Short)
The CoRe is a technology related to the high order modulation, the CoRe balances frequency spectrum energy of each code word bit during consecutive HARQ subpacket retransmission by changing a bit mapping rule within a symbol, so as to average the reliabilities of the code word bits, enhance link performance, and improve system reliability.
Constellation Re-Arrangement Version (CRV for Short)
Constellation re-arrangement version is a concept related to the constellation re-arrangement technology, and is used to indicate constellation mapping rules. One CRV is one kind of the mapping method from a bit sequence to a constellation point.
In the current IEEE 802.16 protocol, bit grouping only includes a bit interleaving operation on a parity bit stream. This method of bit grouping will cause some consecutive bits to have the same reliability level, when there are interferences and noises in a channel, continuous burst errors may occur, which decreases the link performance.
As to the problem in the related art that the link performance decreases as bit grouping makes bits having the same reliability level consecutively distributed, no effective solution has been provided yet at present.